Method of replenishing solutions for coating corrosion resistant alloys



, METHOD OF REPLENISHING SOLUTIONS FOR COATING CORROSION RESISTANT ALLOYS George F. Otto, Oreland, Pa., amignor to Amchem Prodnets, Ind, Ambler, Pa., a corporation of Delaware No Drawing. Filed Oct. 23, 1962, Ser. No. 232,616 6 Claims. (Cl. 1486.24)

The present invention relates to the art of coating corrosion resistant alloys, and is particularly concerned with an improved method of replenishing solutions used in obtaining such coatings.

The term corrosion resistant alloys, as used in this specification as well as in the claims appended hereto, include the various stainless steels, such as those referred to as the 200, 300, 400 and 500 series, the Nichrome and Inconel"= alloys which are nickel based, and various other alloys of two or more of copper, nickel and chromium with or without iron.

Before outlining the objects of the present invention it is desired to refer to certain standard practices in the prior art in order to appreciate more fully the nature of the present invention.

Virtually all commercial processes for coating corrosion resistant alloys are based upon the use of aqueous solutions of oxalic acid. The coatings thus produced are an aid in the chipless deformation of such alloys. Similarly, virtually all of these aqueous oxalic acid solutions are prepared so as to contain therein an accelerating agent, which is usually a sulfur-bearing compound containing oxygen and which will liberate sulfur and/or sulfur dioxide under the acidic conditions of the coating solution.

Such accelerating agents are normally chosen from the class consisting of sulfites, bisulfites, hydrosulfites, thiosulfates and thionates, and are usually employed in the form of an alkali metal salt, particularly the sodium salt. A more detailed understanding of the use of these accelerating agents may be found in the teachings of my issued US. Patents 2,813,816 and 2,953,487, and also in US. Patents 1,911,537, 2,550,660 and 2,758,962.

Commercial installations for coating corrosion resistant alloys are frequently large size tanks, ranging from several hundred to several thousand gallons of coating solution. Over extended periods of time solutions, as above described, have been found to accumulate large quantities of soluble alkali metal ions, particularly sodium ions. Such ions have certain adverse effects upon the operation and maintenance of such oxalate coating solutions as will be hereinafter explained. For example, the accumulation of soluble alkali metal ions results in a buffering action within the coating solution, and acidity control of the bath, which is frequently maintained in a pH range of 0.4 to 1.0, becomes increasingly difiicult.

These alkali metal ions are also suspected of contributing to powdery coating formation on the treated work pieces.

However, of perhaps greatest significance is the deposition of large quantities of crystalline masses within the coating solution tanks, particularly when the coating bath has been allowed to cool to ambient temperatures, as occurs during normal Weekend plant shut-down. These crystalline deposits have been found to consist primarily of sodium acid oxalate (NaHC O l-l O), which is very difficult to dissolve from the crystalline state. This crystallization problem results in the removal of large quantities of the oxalic acid coating constituent from the treating solutions, thereby causing appreciable loss of acidity and compounding the problems of bath acidity control and replenishment.

*Trademarks for nickel alloys as described in Engineering Alloys (Woldman) published by ASTM, 1954 Edition.

3,218,201 Patented Nov. 16, 1965 This crystallization problem has been so severe in several heavily used commercial installations, that as much as one-half of the volume of the treating solutions, after having cooled to room temperature, has been observed to be a solid crystalline mass. Such crystalline deposition causes serious impediments in the treating tanks by virtue of the size and nature of these needle-like crystals, which grow from the sides and bottom of the tank thereby obstructing and impeding the free movement of work pieces therein.

A recent attempt to solve this problem involve a suggestion for using only ammonium salts of the sulfurbearing, oxygen containing accelerating agents, since ammonium acid oxalate is more soluble than the corresponding sodium salt in the acidic coating solutions. However, while this proposal offers some relief to the problem at hand, it falls short of providing adequate relief due to the fact that an appreciable amount of ammonium acid oxalate salt is still precipitated from the treating solution.

With the foregoing in mind, the principal object of the present invention may be said to reside in the provision of an improved method for replenishing the sulfur-bearing, oxygen containing accelerating ion content of aqueous oxalic acid coating solutions, which substantially minimizes or essentially overcomes the formation of large needle-like crystals in the treating tanks thereby avoiding the concomitant loss of acidity from precipitation of the essential coating constituents.

Other objects and advantages of the present invention will become apparent from a consideration of the following detailed disclosure.

The present invention is based upon the discovery that if the sulfur-bearing oxygen containing accelerating agents selected from the class consisting essentially of sulfites, bisulfites, hydrosulfites, thiosulfates and thionates, used for purposes of replenishing aqueous acid oxalate coating solutions, are added thereto as salts of certain divalent cations, the oxalate salts of which cations are substantially insoluble in the aqueous acid coating solutions, marked improvements will result in the behavior of the coating solutions with substantially reduced loss of the essential oxalate coating ions through precipitation therefrom.

Hereinafter in this specification, as well as in the claims appended hereto, use of the expression substantially insoluble shall refer to a solubility of certain divalent cation salts of oxalic acid of less than 1 gram/liter in the acid oxalate coating solutions.

The divalent cations which may be combined with the sulfur-bearing oxygen containing accelerating agents to form salts thereof, and which cations form substantially insoluble oxalates in the acidic coating solutions, are selected from the class consisting essentially of calcium, magnesium, barium, cadmium, zinc, manganese and ferrous iron.

A number of these salts are known commercially, such as for example, calcium sulfite or bisulfite, and zinc hydrosulfite. Moreover, these salts may easily be prepared from the oxides, hydroxides, chlorides or carbonates of the divalent cations by reacting same with sulfur dioxide, or with alkali metal salts of the desired accelerator anion. For example, calcium sulfite may be prepared from the interaction of sodium bisulfite with calcium hydroxide, and manganese or zinc bisulfite may be obtained by reacting the metallic carbonates with gaseous sulfur dioxide.

Although the method of preparation of several of these divalent cation salts of the sulfur-bearing oxygen containing accelerating compounds have been indicated, such preparations form no part of the present invention, and other methods, not listed herein, may be utilized in accordance with general chemical principles.

The amount of divalent cation salts of oxalate coating accelerating agents from the class described, which must be employed in order to obtain the desired coating results depends entirely upon the degree of bath loading. Since such. amounts are determined in accordance with prior art practice, no detailed teachings with respect to this usage are presented herewith.

So far as concerns the actual incorporation of these divalent cation salts, from the class described, into the oxalate coating solutions, this may be accomplished by the use of aqueous solutions or slurries of such salts, or simply by adding the requisite amount of a powdered or dry salt to the coating bath. Generally, it has been found that aqueous solutions or slurries are easier to handle, although with the exception of the bisulfite salts, the majority of these salts of accelerator agents are obtainable in solid or crystalline form due to their limited solubility in aqueous media.

In order to illustrate the improved results secured in accordance with the teachings of this invention there is presented below a detailed outline of a process run in accordance with the replenishing technique of this invention.

Stainless steel panels (type 304) were pickled in a nitric-hydrofluoric acid bath according to well established art practice. After water rinsing, these panels were sub jected to the coating action of the solution described below using 5 minute immersion cycles at 180 Grams Oxalic acid 4O Boric acid 20 Sodium thiosulfate 2.0

Water, to make 1 liter such solution being described and claimed in my issued US. Patent 2,953,487.

After 1 square foot of total surface area had been treated in this solution, bath analyses (as determined by titrating a ml. sample of the coating solution with 0.05 N iodine solution to a blue endpoint using starch indicator) showed a deficiency of accelerating agent, and 0.35 gram/liter of calcium bisulfite was added in the form of an aqueous solution thereof. Additional steel panels were processed through the coating solution which was replenished, as required with oxalic and boric acids, and with calcium bisulfite, until a total of 25 square feet of steel had been coated. inspection or" the coated panels showed fine crystalline coatings having weights of 426 mm/sq. foot.

In order to compare the foregoing with the standard replenishing technique of the prior art, a coating solution was prepared containing:

Grams Oxalic acid 4O Boric acid Sodium thiosulfate 2.6

Water, to make 1 liter such solution being exactly similar to the bath prepared for the experiment reported above.

Stainless steel (type 304) panels, pickled in nitrichydrofiuoric acid as before, were water rinsed and then subjected to the action of the oxalate coating solution using immersion times of 5 minutes at 180 F. Replenishment of the coating constituents was accomplished using oxalic and boric acids as before. However, the accelerating agent was supplied as sodium thiosulfate throughout the entire coating operation. After square feet of total surface area has been treated, the steel panels were found to contain fairly coarse crystalline coatings having weights or" 533 mg./sq. foot.

Both coating solutions were permitted to cool, and were then allowed to remain undisturbed at ambient temperatures for i5 hours. Subsequent inspection of the baths showed a solid /s inch layer of large needle-like crystalline deposits in the solution replenished with sodium thiosuliate; but only a /8 inch layer of a line, powdery non-adherent type precipitate which was freely distributed on the bottom of the coating solution tank in the solution replenished with calcium bisultite.

Analyses of thecrystalline deposit obtained from the sodium thiosulfite accelerated bath indicated the precipitated crystals were almost entirely hydrated sodium acid oxalate (NaHC OJ-I O). Analyses of the line powder from the calcium bisulfite accelerated bath indicatcd it to be about 32180 about Ctr-C 6, and about 22% FeC Q The balance of the fine powdery deposit was sulfur and iron oxides.

Comparison of the results obtained from these two processes clearly shows that the use of a salt of an accelerating agent, cation portion of which forms a substantially insoluble oxalate salt in oxalic acid coating solutions, successfully avoids the serious crystallization problems encountered in prior art processes. It has also been oberved that the use of calcium salts of accelerating agents yields a refinement in the coatings obtained on the metal work pieces. For this reason, it may sometimes be desirable to employ Zinc, cadmium, ferrous or manganese salts of the accelerating agents, since this refining elfect has not been observed from the use of these cations.

I claim:

1. in the replenishment of the accelerator ion content of aqueous oxalic acid coating solutions, said accelerator ion containing sulfur and oxygen and being selected from the class of anions consisting of sulfitcs, bisulfites, hydrosulfites, thiosulfates and thionates, the improvement which comprises adding such accelerator anions in association with a cation selected from the class consisting of calcium, magnesium, barium, cadmium, zinc. manganese and ferrous iron.

2. The method of claim it wherein the cation is calcium.

3. The method of claim it wherein the cation is zinc. The method of claim 1 wherein the cation is ferrous iron.

5. The method of claim 1 wherein the cation is magnesium.

e. The method of claim 1 wherein the cation is main ganesc.

References -Cited by the Examiner FOREIGN PATENTS 3/1955 Austria.

RICHARD D. NEVIUS, Primary Examiner.

,VELLIAM D. MARTIN, Examiner. 

1. IN THE REPLENISHMENT OF THE ACCELERATOR ION CONTENT OF AQUEOUS OXALIC ACID COATING SOLUTIONS, SAID ACCELERATOR ION CONTAINING SULFUR AND OXYGEN AND BEING SELECTED FROM THE CLASS OF ANIONS CONSISTING OF SULFITES, BISULFITES, HYDROSULFITES, THIOSULFATES AND THIONATES, THE IMPROVEMENT WHICH COMPRISES ADDING SUCH ACCELERATOR ANIONS IN ASSOCIATION WITH A CATION SELECTED FROM THE CLASS CONSISTING OF CALCIUM, MAGNESIUM, BARIUM, CADMIUM, ZINC, MANGANESE AND FERROUS IRON. 